Blower filter systems are used for light and medium respirators and support the user of respirator filters by reducing the breathing resistance, contrary to conventional gas masks, thus making long fatigue-free use possible. A blower filter system comprises the following main components: A blower filter device, usually worn on the belt, and a head piece, which is designed as a hood or mask. These two components are usually connected to one another via a connection tube. The contaminated air is drawn through a filter by means of the blower filter device, as a result of which it is freed from harmful substances. The purified air is subsequently sent to the head piece and fed to the user of the respirator.
The blower filter device has, among other things, a blower unit with a fan impeller driven by a motor and with a (spiral) housing. The power for the blower unit is usually supplied by a battery. In addition, the blower filter device has a central control unit, which controls the motor of the blower unit and can process inputs of the user. A housing encloses, in general, the blower unit, the control unit and the battery. At least one filter can be connected to the housing.
Blower filter systems are used under greatly different ambient conditions. This may also comprise situations with potentially explosive gases or dusts, which impose special requirements on the components used. Two causes of a potential ignition must be ruled out in case of explosion protection: On the one hand, spark ignition, usually prevented by limiting the energy being released in case of a defect, and, on the other hand, a temperature limitation, which rules out self-ignition.
If a jack is provided at the battery or the blower filter device for charging the battery, this jack must be isolated to avoid spark or glow discharges. This is usually brought about by the use of diodes connected in series, which are designed as single or double redundant diodes, depending on the margin of error needed. One to three diodes, through which the charging current flows, are thus obtained at the charging terminal.
Short charging times are advantageous for users of blower filter devices to achieve the best possible utilization of a blower filter system. It is necessary for this to apply correspondingly high charging currents. Blower filter devices often have batteries or battery packs comprising one or more cells with an energy content of 50 Wh to 70 Wh. Assuming a usual voltage of 14.4 V, this corresponds to approx. 3.5 Ah to 5 Ah. If charging shall be completed in one hour, a current of 3.5 A to 5 A, on average, must be applied over one hour.
The drawback arising from the prior-art solutions with 1-3 diodes connected in series in the charging circuit is the power loss generated in the diodes during charging. This equals between 2.5 W and 7.5 W in the above-mentioned scenario. These large quantities of heat may lead to damage to the battery or the battery pack and cannot therefore be implemented.
DE 10 2005 059 571 A1 discloses an electric circuit, which makes it possible to charge a battery inductively in a small appliance, such as a toothbrush. The circuit contains a charging circuit for charging a battery, which has a charging coil for generating an alternating current and a rectifier diode, wherein the battery is connected in series with the rectifier diode and the charging coil. However, it is difficult to charge batteries for blower filter devices inductively because of the low efficiency of power transmission.